Aggregation framework summary ：
Before the formal conclusion , Just look at the difference between a set and an array
The array is fixed , The set is variable ; Arrays can be basic types , It can also be a reference type , But collections can only be reference types ; Arrays can only store the same type , Collections can store different types of ( Usually the collection stores the same type ).
Orderly , repeatable , Multiple can be inserted null Elements
The bottom layer is array structure , Thread unsafe , Efficient , Quick query , Add or delete slowly .
The bottom layer is linked list structure , Thread unsafe , Efficient , Additions and deletions quickly , Slow query , And the operation of the first and last elements is easy , Generally, you can directly add 、 Delete 、 Get the first and last elements .
The bottom layer is array structure , Thread safety , Same as ArrayList equally , Quick query , Add or delete slowly , But it's inefficient .
disorder , Do not repeat , Only one deposit is allowed null Elements
The bottom layer is implemented by hash table ,
Key value pairs are stored in form , That is to say key-value form , Store the mapping between keys and values , disorder , only , Key is not repeatable ,